1. Field of the Invention
The present invention relates to a laser apparatus and a method of controlling the laser apparatus.
2. Description of the Related Art
Realization of a high output of a laser instrument has progressed over time. Particularly, a solid laser (typified by a Nd:YAG laser and so on using a laser medium obtained by adding neodymium (Nd) to Y3Al5O12 crystal as a base material) is suitable for high energy applications. A noble-gas flash lamp and a semiconductor laser (LD) are used as an excitation source for a solid laser, and the noble-gas flash lamp has the advantages that it is inexpensive and that energy output per pulse can be increased. Thus, the noble-gas flash lamp is widely used for industrial applications such as a laser light machine and a laser annealing apparatus.
Recently, the development of photoacoustic measurement apparatus for medical use using laser light has progressed (NPL 1, below). In such an apparatus, a diagnosis of the presence of a tumor in the breast using the photoacoustic effect has been attempted. The photoacoustic measurement apparatus is a measurement apparatus which irradiates a portion of tissue (a “measured portion”) with a nanosecond pulse laser to receive an ultrasound wave (photoacoustic wave) generated in the measured portion, and, thus, to analyze a received signal, whereby an image is obtained. Especially, in order to obtain a photoacoustic signal from a relatively deep biological portion, laser light of high energy output per pulse is required, and a lamp excitation solid laser is suitably used.
However, since the emission spectrum of a flash lamp is as wide as from the ultraviolet region to the infrared region, the flash lamp is less consistent with an absorption spectrum of a laser medium. Thus, a thermal load is applied to the laser medium, and there is the disadvantage that beam quality deteriorates due to the thermal lens effect, the thermal birefringence effect, and so on. Especially, the output energy at the initial stage of rising of the laser oscillation is unstable until the laser medium reaches thermal equilibrium. Thus, since the laser oscillation output gradually changes, a problem occurs in processing and so on.
To respond to the above problem, there is disclosed a method of measuring the laser output for each pulse to control a lamp current supplied from a power supply to a flash lamp so that each pulse energy is constant, and, thus, to obtain a stable output (PTL 1, below).
Flash lamps have different characteristics from each other. As the flash lamp is used, the emission intensity is reduced, and the flash lamp eventually needs to be exchanged. The consumption of the flash lamp strongly affects the convenience of a lamp excitation solid laser. To respond to this problem, there is disclosed a method of detecting the laser output as needed by a photodetection element, then, when the lamp current value is more than a predetermined upper limit value or lower limit value, displaying the fact, and indicating the need to exchange the flash lamp by the display (PTL 2, below).
In the laser apparatus using the lamp excitation solid laser, it is very important to control a lamp setting value to enhance the convenience.    PTL 1: Japanese Patent No. 2542821    PTL 2: Japanese Patent Application Laid-Open No. 8-195521    NPL 1: S. Manohar et al., Proc. of SPIE, vol. 6437, 643702-1